A) Field of Invention
This invention relates generally to a diesel exhaust aftertreatment system and more particularly to a diesel exhaust treatment system that simultaneously provides for a high level reduction of nitrogen oxides (NOx) and particulate emissions under lean engine operating conditions.
B) Incorporation by Reference
The following United States patents are incorporated by reference herein and made a part hereof. Specifically, the compositions of the catalysts disclosed in the patents below and how the compositions are made and/or applied to the disclosed filter or SCR catalysts are incorporated herein so that such material need not be repeated or explained again in detail in the Detailed Description of this invention.                U.S. Pat. No. 4,833,113 to Imanari et al., issued May 23, 1989, entitled “Denitration Catalyst for Reducing Nitrogen Oxides in Exhaust Gas”;        U.S. Pat. No. 4,961,917 to Byrne, issued Oct. 9, 1990, entitled “Method for Reduction of Nitrogen Oxides with Ammonia Using Promoted Zeolite Catalysts”;        U.S. Pat. No. 5,100,632 to Dettling et al., issued Mar. 31, 1992, entitled “Catalyzed Diesel Exhaust Particulate Filter”; and,        U.S. Pat. No. 5,804,155 to Farrauto et al., issued Sep. 8, 1998, entitled “Basic Zeolites as Hydrocarbon Traps for Diesel Oxidation Catalysts”.        
While the catalysts disclosed in the patents incorporated by reference herein may be used in the present invention, they do not, per se, or, in and of themselves, form the present invention.
C) Prior Art
Compression ignition diesel engines have great utility and advantage as vehicle power plants because of their inherent high thermal efficiency (i.e. good fuel economy) and high torque at low speed. Diesel engines run at a high A/F (air to fuel) ratio under very fuel lean conditions. Because of this they have very low emissions of gas phase hydrocarbons and carbon monoxide. However, diesel exhaust is characterized by relatively high emissions of nitrogen oxides (NOx) and particulates. The particulate emissions, which are measured as condensed material at 52° C., are multi phase being comprised of solid (insoluble) carbon soot particles, liquid hydrocarbons in the form of lube oil and unburned fuel, the so called soluble organic fraction (SOF), and the so called “sulfate” in the form of SO3+H2O=H2SO4.
Both NOx and particulates have been difficult diesel exhaust components to convert and future emissions standards have been recently adopted in the US and Europe for both heavy duty and light duty diesel powered vehicle which are expected to require reduction of both of these emissions by at least 50% and quite likely by 70–90%.
One commercial aftertreatment technology which has proven very successful for reduction of NOx under lean exhaust conditions for stationary sources is Selective Catalytic Reduction (SCR). In this process NOx is reduced to N2 with NH3 over a catalyst (e.g. zeolite, V/Ti). This technology is capable of NOx reduction in excess of 90% and thus it is one of the best candidates for meeting the aggressive NOx reduction goals. SCR is currently under development for mobile source, vehicle applications using urea (e.g. aqueous solution) as the source of NH3. SCR is very efficient for NOx reduction as long as the exhaust temperature is within the active temperature range of the catalyst (e.g. >300° C.). Unfortunately diesel exhaust temperatures are many times considerably lower than that required for good catalyst efficiency (i.e., below “light-off”). This is especially true for light duty (LD) diesel applications such as diesel autos which operate at light load for the most part, resulting in very low exhaust temperatures (150–250° C.). Even diesel trucks operate under conditions which result in exhaust temperatures below the optimum temperatures for SCR catalysts. Unfortunately, one of the best, most stable, SCR catalysts, which is of the zeolite type (e.g. The assignee, Engelhard Corporation's ZNX catalyst, a Fe-Beta Zeolite), also has the highest optimum operating temperature. As a result its effectiveness is greatly diminished at diesel exhaust temperatures of interest.
One key aftertreatment technology under development for very high level particulate reduction is the diesel particulate filter. There are many known filter structures that can be used to remove particulates from diesel exhaust, including honeycomb wall-flow filters, wound or packed fiber filters, open-cell foams, sintered metal filters, etc. However, ceramic wall-flow filters have received the most attention. These filters are capable of removing over 90% of the particulate material from diesel exhaust and thus can meet this emissions reduction goal. The filter is a physical structure for removing particles from exhaust and the accumulating particles will increase the back pressure from the filter on the engine. Thus the accumulating particles (soot+hydrocarbons) have to be continuously or periodically burned out of the filter to maintain an acceptable backpressure level. Unfortunately, the carbon soot particles require temperatures in excess of 500–550° C. to be combusted under oxygen rich (lean) exhaust conditions. This is higher than typical diesel exhaust temperatures. A means must be provided to lower the soot burning temperature in order to provide for “passive” regeneration of the filter. One good way to accomplish this is to provide a suitably formulated catalyst which is applied to the filter. The presence of the catalyst has been found to provide soot combustion and thereby regeneration of the filter at temperatures accessible within the diesel engine's exhaust under realistic duty cycles. In this way a Catalyzed Soot Filter (CSF) or Catalyzed Diesel Particulate Filter (CDPF) can be an effective way to provide for >90% particulate reduction along with passive burn-out of the accumulating soot and thereby filter regeneration.
In stationary applications, a number of arrangements routinely use filters upstream of an SCR catalyst with an ammonia reductant injected between filter and SCR catalyst. Several arrangements are disclosed in Nitrogen Oxides Control Technology Fact Book, 1992, Noyes Data Corporation, pages 84–105. However, all the temperatures for SCR are high and the filters, discussed generally, are of the dust particulate type such as electrostatic precipitators.
Hug Engineering AG has developed a gas purification stationary system described in SAE paper 930363, “Off-Highway Exhaust Gas After-Treatment Combining Urea-SCR, Oxidation Catalysis and Traps”. In this system, NH3 is injected upstream of catalyst beds containing an SCR followed by an oxidation catalyst. In a later Hug brochure (1996) a soot filter bed (optional) is provided in a casing adjacent to and upstream of a SCR reactor adjacent to and upstream of an oxidation catalyst and the urea injected into the waste gases passing through, in sequence, the filter, SCR and oxidation catalyst. The soot filter is described as a fibrous bundle which filters fine soot particles from the exhaust stream that have a carcinogenic effect. The Hug system disclosed has been applied to a ferry and other large diesel engine applications operating for the most part at steady speeds and higher temperatures than the vehicular applications of the present invention. A Hug brochure for stationary gas purification systems describes Hug's “Staru” system in which the soot filter is split from the SCR and oxidation catalysts with NH3 injected therebetween. The soot filter described as fibrous to continue the function of trapping fine soot particles but is catalytically coated to regenerate or burn off the soot at 450° C. In general, the Hug systems have shown the ability to reduce NOx exhaust emissions from large diesel engines operating generally steady state at higher temperatures than light duty diesel engines by injecting NH3 upstream of SCR-oxidation catalysts and using a downstream fibrous, regeneration filter to trap fine soot particles.
The patent literature discloses U.S. Pat. No. 5,746,989 to Murachi et al. issued May 5, 1998 and PCT application PCT/GB99/03281 (published Apr. 20, 2000 as WO 00/21647) which use NOx absorbers that are periodically regenerated. Downstream of the NOx absorber is an oxidation catalyst and between absorber and oxidation catalyst is a particulate filter. In the '989 patent, the absorber is regenerated by varying the A/F ratio and in the PCT application, NOx reactant is injected upstream of the absorber.
U.S. Pat. No. 4,912,776 to Alcorn issued Mar. 27, 1990 discloses an oxidation catalyst, an SCR catalyst downstream and adjacent to the oxidation catalyst, and a reductant source introduced to the exhaust between the oxidation catalyst and the SCR catalyst. Consistent with at least one of the theories of the present invention, the Alcorn concept is believed to produce improved NOx reduction. A variation of Alcorn is disclosed in PCT application NO. PCT/GB99/00292 (published Aug. 12, 1999 as WO 99/39809) in which upstream of Alcorn's oxidation catalyst is placed a particulate filter and the source of reductant is positioned downstream of the SCR catalyst and upstream of the particulate filter. The particulate filter is disclosed as a wall-flow filter effective to cause “combustion” at relatively low temperatures in the presence of NO2 which is not believed especially beneficial in the arrangement disclosed in the PCT application. U.S. Pat. No. 4,902,487 to Cooper et al. issued Feb. 20, 1990 should also be noted for its disclosure of a particulate filter upstream of a platinum based catalyst which arrangement is said to generate NO2 from the exhaust gas.